Program failure recovery

ABSTRACT

Methods and apparatus are provided. A method of operating a memory device includes detecting a programming failure at a first location of a memory array, preserving data within the memory device when the program failure is detected, programming a second location of the memory array with a first portion of the preserved data, programming a third location of the memory array with a second portion of the preserved data, and combining, at the second location of the memory array, the first portion of the preserved data programmed in the second location of the memory array with a third portion of the preserved data from the first location of the memory array.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 10/431,767 (pending), filed May 8, 2003 and titled “PROGRAM FAILURE RECOVERY,” which application is commonly assigned and incorporated by reference in its entirety herein.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and in particular the present invention relates to the operation of memory devices when a program failure occurs.

BACKGROUND OF THE INVENTION

A flash memory device is a type of electrically erasable programmable read-only memory (EEPROM) and is used for non-volatile storage of data. Flash memory is being increasingly used to store execution codes and data in portable electronic products, such as computer systems.

A typical flash memory comprises a memory array having rows and columns of memory cells. Each of the memory cells is fabricated as a field-effect transistor having a control gate and a floating gate. The floating gate is capable of holding a charge and is separated by a thin oxide layer from source and drain regions contained in a substrate. Each of the memory cells can be electrically programmed (charged) by injecting electrons from the drain region through the oxide layer onto the floating gate. The charge can be removed from the floating gate by tunneling the electrons to the source through the oxide layer during an erase operation. Thus, the data in a memory cell is determined by the presence or absence of a charge on the floating gate.

NOR and NAND flash memory devices are two common types of flash memory devices, so called for the logical form the basic memory cell configuration in which each is arranged. Typically, for NOR flash memory devices, the control gate of each memory cell of a row of the array is connected to a word-select line, and the drain region of each memory cell of a column of the array is connected to a bit line. The memory array for NOR flash memory devices is accessed by a row decoder activating a row of floating gate memory cells by selecting the word-select line coupled to their gates. The row of selected memory cells then place their data values on the column bit lines by flowing a differing current if in a programmed state or not programmed state from a coupled source line to the coupled column bit lines.

The array of memory cells for NAND flash memory devices is also arranged such that the control gate of each memory cell of a row of the array is connected to a word-select line. However, each memory cell is not directly coupled to a column bit line by its drain region. Instead, the memory cells of the array are arranged together in strings, typically of 16 each, with the memory cells coupled together in series, source to drain, between a source line and a column bit line. The memory array for NAND flash memory devices is then accessed by a row decoder activating a row of memory cells by selecting the word-select line coupled to a control gate of a memory cell. In addition, the word-select lines coupled to the control gates of unselected memory cells of each string are driven to operate the unselected memory cells of each string as pass transistors, so that they pass current in a manner that is unrestricted by their stored data values. Current then flows from the source line to the column bit line through each series coupled string, restricted only by the selected memory cells of each string. This places the current-encoded data values of the row of selected memory cells on the column bit lines.

Many NAND flash memory devices provide two sets of registers (or latches) for use during program operations. These are usually referred to as cache latches and program latches. During programming operations, data are transferred from a host, such as a processor of a portable electronic product, into the cache latches. The data are then transferred from the cache latches into the program latches prior to the actual programming of a row of memory cells (commonly referred to as a page). This frees up the cache latches for additional data transfer from the host, while programming of the current data continues using the program latches in what is referred to as cache program operation.

In the event of a program failure, data transfer from the cache to program latches continues and the programming of this data in its specified location (or page) is still performed so that program data no longer remains within the flash device after a program operation has failed. A RAM buffer is often located externally to the flash device to retain the program data in case of failure. This data can be sent again to the flash device for programming in a new location (or page) as part of a recovery to ensure storage of the data.

The buffer size required for storing program data in case of failures is dictated by the page size of the flash memory. Therefore, as page size increases, e.g., due to increasing memory requirements, buffer size and thus buffer cost will increase.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives to using buffers for storing program data in case of failures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flash memory system according to an embodiment of the present invention.

FIGS. 2A-2D illustrate data transfer during conventional programming of a memory array of the flash memory of FIG. 1.

FIGS. 3A-3D illustrate data transfer during a method of operating a memory device when a program failure occurs according to an embodiment of the present invention.

FIG. 4 is a flowchart of a method of operating a memory device when a program failure occurs according to another embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.

FIG. 1 is a block diagram of a flash memory system 100 according to an embodiment of the present invention. Flash memory system 100 includes a memory (or mass storage) device 102, such as a NAND flash memory device, coupled to a processor or data controller 104. For one embodiment, memory device 102 includes an array 106 of individual storage locations (or memory cells) 105, e.g., flash memory cells, each cell having a distinct memory address. Array 106 is arranged in rows and columns, with the rows arranged in addressable blocks. For one embodiment, array 106 has rows (or pages) 107 ₁ to 107 _(N), as shown, where each of rows (or pages) 107 ₁ to 107 _(N) includes a plurality of memory cells 105. In other words, a plurality of memory cells 105 comprises one of rows (or pages) 107 ₁ to 107 _(N). Memory system 100 has been simplified to focus on features of the memory that are helpful in understanding the invention.

Data stored in the memory array 106 can be accessed using externally provided location addresses received by address latches 108 via a plurality of data (DQ) lines 124. Address signals are received and decoded to access the memory array 106. Sense amplifier and compare circuitry 112 is used to sense data stored in the memory cells and verify the accuracy of stored data. Command control circuit 114 decodes signals provided on control link 116 from the controller 104 and controls access to the memory cells of array 106. These signals are used to control the operations of the memory, including data read, data write, and erase operations.

A data input buffer circuit 120 and a data output buffer circuit 122 are included for bi-directional data communication over the plurality of data (DQ) lines 124 with the controller 102. For one embodiment, data input buffer circuit 120 includes cache latches (or data registers) 130. For another embodiment cache latches 130 include cache-latch cells 132 respectively corresponding to memory cells 105 of each of rows 107 ₁ to 107 _(N). Memory device 102 also includes program latches (or data registers) 140. For one embodiment, program latches 140 include program-latch cells 142 respectively corresponding to cache-latch cells 132 and of memory cells 105 of each of rows 107 ₁ to 107 _(N). For another embodiment, cache latches 130 are serially connected to program latches 140.

To program array 106, command control circuit 114 decodes program commands received from data controller 104. Programming of array 106 includes selecting a location within array 106 to program, e.g., row (or page) 107 ₂ of array 106, as shown in FIG. 1. FIGS. 2A-2D illustrate data transfer during conventional programming of array 106. Data for row 107 ₂ are transferred from controller 104 to cache latches 130, as shown in FIG. 2A. After the data for row 107 ₂ are transferred to cache latches 130, these data are transferred from cache latches 130 to program latches 140, as shown in FIG. 2B, and programming of row 107 ₂ commences. During programming of row 107 ₂, another row of array 106 can be selected and data for that row can be transferred into cache latches 130 from controller 104, as shown in FIG. 2C. Also during programming of row 107 ₂, the data for row 107 ₂ are transferred from program latches 140 to row 107 ₂, and the contents of program latches 140 are altered, as shown in FIG. 2D, e.g., returned to that of FIG. 2A.

Programming of row 107 ₂ with the data of program latches 140 is accomplished by combining the data of row 107 ₂ with the data of program latches 140 using a logical AND operation. For example, a zero (0) of the data of program latches 140 combined with a corresponding one (1) of the data of row 107 ₂ using a logical AND causes a zero (0) to replace the one (1) in row 107 ₂. A one (1) of the data of program latches 140 combined with a corresponding one (1) of the data of row 107 ₂ using a logical AND produces a one (1) in row 107 ₂. For one embodiment, programming row 107 ₂ involves replacing the ones (1s) of row 107 ₂ with the corresponding zeros (0s) of program latches 140.

The data are typically verified as they are transferred to row 107 ₂, e.g., to determine if the data transferred to row 107 ₂ matches the data previously in program latches 140. Note that the data in program latches 140 of FIG. 2C matches the data in row 107 ₂ of FIG. 2D, indicating successful programming of row 107 ₂. In one embodiment, as each memory cell 105 of row 107 ₂ is programmed successfully (or verified) a one (1) is placed in a corresponding program-latch cell 142 of program latches 140. Note that each of the program-latch cells 142 of program latches 140 has one (1) in FIG. 2D, indicating that each of the corresponding memory cells 105 of row 107 ₂ is verified and thus indicating successful programming of row 107 ₂.

If the data transferred to row 107 ₂ does not match the data previously in program latches 140, a program failure occurs. This is illustrated by comparing the data in program latches 140 of FIG. 2C to the data in row 107 ₂ (the failed location) of FIG. 3A. For one embodiment, when a memory cell of row 107 ₂ fails to program (or verify) a zero (0) is placed in the corresponding program-latch cell 142 of program latches 140. For embodiments where programming row 107 ₂ involves replacing the ones (1 s) of row 107 ₂ with the corresponding zeros (0s) of program latches 140, a program failure involves retaining a zero (0) in the program-latch cell 142 corresponding to the memory cell 105 of row 107 ₂ where the failure occurred. Note that a zero (0) appears in program-latch cell 142 in program latches 140 in FIG. 3A indicative of the program failure in row 107 ₂.

FIGS. 3A-3D illustrate data transfer within memory device 102 during a method 400, according to an embodiment of the present invention, of operating memory device 102 when a program failure is detected. For various embodiments, detecting a program failure involves command control circuit 114 detecting a zero (0) in program latches 140. A flowchart of method 400 is presented in FIG. 4. Command control circuit 114 is adapted to perform method 400 when a program failure is detected. Method 400 preserves the data within memory device 102 at block 410 when a program failure is detected. This is accomplished by stopping programming operations in progress before the program failure occurs when the program failure is detected. Stopping programming operations includes stopping the transfer of data to row 107 ₂ from program latches 140 as indicated by a slash 300 passing through arrow 302 of FIG. 3A, and stopping altering the data within program latches 140. This preserves the data contained within row 107 ₂ and program latches 140 at the time of the program failure. Stopping programming operations can also include stopping data transfer from controller 104 to cache latches 130 if data are being transferred from controller 104 to cache latches 130 when the program failure is detected and in another embodiment, stopping data transfer from cache latches 130 to program latches 140 if data are being transferred from cache latches 130 to program latches 140 when the program failure is detected. This preserves the data contained within cache latches 130 at the time of the program failure.

At block 420, row (or page) 107 _(i), for example, is programmed using the data contained in program latches 140 at the time of the program failure. This includes selecting row 107 _(i) and transferring the data from program latches 140 to row 107 _(i), as shown in FIG. 3B.

At block 430, row (or page) 107 _(i+1), for example, is programmed using the data contained in cache latches 130 at the time of the program failure. This includes selecting row 107 _(i+1), transferring the data from cache latches 130 to program latches 140, and transferring the data from program latches 140 to row 107 _(i+1), as shown in FIG. 3C.

At block 440 failed data from row 107 ₂ is combined with the data stored in row 107 _(i), e.g., by programming or copying the failed data from row 107 ₂ on top of the data stored in row 107 _(i). This reconstructs the data originally intended for row 107 ₂ at row 107 _(i) before the program failure, as shown in FIG. 3D. Note that the data in row 107 _(i) of FIG. 3D match those in program latches 140 (the data originally intended for row 107 ₂ before the program failure depicted in FIG. 3A) in FIG. 2C.

To combine failed data from row 107 ₂ with the data stored in row 107 _(i), the failed data are transferred from row 107 ₂ to cache latches 130, as shown in FIG. 3D. The failed data are then transferred from cache latches 130 to program latches 140 and subsequently to row 107 _(i). Transferring the failed data from program latches 140 to row 107 _(i) involves programming row 107 _(i) with the failed data from program latches 140. For one embodiment, the failed data from row 107 ₂ are combined with the data stored in row 107 i using a logical AND operation as described above.

For one embodiment, row 107 ₂ is assigned a defective status and is treated as a defect to avoid accessing the failed data therein during operation of memory device 102.

CONCLUSION

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof. 

1. A method of operating a memory device, the method comprising: detecting a programming failure at a first location of a memory array; preserving data within the memory device when the program failure is detected; programming a second location of the memory array with a first portion of the preserved data; programming a third location of the memory array with a second portion of the preserved data; and combining, at the second location of the memory array, the first portion of the preserved data programmed in the second location of the memory array with a third portion of the preserved data from the first location of the memory array.
 2. The method of claim 1, wherein preserving data within the memory device when the program failure is detected comprises preserving the first portion of the preserved data at a first data register and preserving the second portion of the preserved data at a second data register.
 3. The method of claim 1, wherein preserving data within the memory device when the program failure is detected comprises stopping programming operations within the memory device in progress before detecting the programming failure.
 4. The method of claim 1, wherein preserving data within the memory device when the program failure is detected comprises stopping a transfer of contents of a register to the first location of the memory array.
 5. The method of claim 1, wherein preserving data within the memory device when the program failure is detected comprises stopping altering contents of a data register.
 6. The method of claim 1, wherein the first and third portions of the preserved data are combined at the second location of the memory array using a logical AND operation.
 7. The method of claim 1, wherein preserving data within the memory device when the program failure is detected comprises stopping receiving data at the memory device from a location external to the memory device.
 8. The method of claim 1, wherein detecting a programming failure comprises detecting a predetermined data value in the first portion of the preserved data.
 9. The method of claim 1, wherein programming a third location of the memory array with a second portion of the preserved data comprises transferring the second portion of the preserved data from a first data register to a second data register and from the second data register to the third location of the memory array.
 10. A method of operating a memory device, the method comprising: detecting a programming failure at a first location of a memory array; preserving data when the program failure is detected at first and second data registers and at the first location of the memory array; programming a second location of the memory array with a first portion of the preserved data from the first register; programming a third location of the memory array with a second portion of the preserved data from the second register; and combining, at the second location of the memory array, the first portion of the preserved data programmed in the second location of the memory array with a third portion of the preserved data from the first location of the memory array.
 11. The method of claim 10, wherein programming a third location of the memory array with a second portion of the preserved data from the second register comprises: transferring the second portion of the preserved data from the second data register to the first data register; and transferring the second portion of the preserved data from the first data register to the third location of the memory array.
 12. The method of claim 10, further comprising receiving data into the second data register from a location external to the memory device before the programming defect is detected.
 13. A method of operating a mass storage device having at least two serially-connected data input latches coupled to a storage array, the method comprising: upon detecting a failure of a programming operation of a first location of the storage array using first data from a first data input latch: programming a second location of the storage array using second data from the first data input latch; transferring third data to the first data input latch from the second data input latch; transferring the third data into a third location of the storage array from the first data input latch; transferring the failed data from the first location of the storage array to the second data input latch; transferring the failed data to the first data input latch from the second data input latch; and combining the failed data from the first data input latch with the second data in the second location of the storage array.
 14. The method of claim 13, wherein the failed data is combined with the second data in the second location of the storage array programming using a logical AND operation.
 15. A flash memory device comprising: an array of flash memory cells; and a command control circuit for controlling access to the array of flash memory cells, wherein the command control circuit is adapted to perform a method comprising: detecting a programming failure at a first location of the array; preserving data within the memory device when the program failure is detected; programming a second location of the array with a first portion of the preserved data; programming a third location of the array with a second portion of the preserved data; and combining, at the second location of the memory array, the first portion of the preserved data programmed in second location of the memory array with a third portion of the preserved data from the first location of the memory array.
 16. The flash memory device of claim 15, wherein, in the method, the first and third portions of the preserved data are combined at the second location of the memory array using a logical AND operation.
 17. The flash memory device of claim 15, wherein, in the method, detecting a programming failure comprises detecting a predetermined data value in the first portion of the preserved data.
 18. The flash memory device of claim 15, wherein, in the method, preserving data within the memory device when the program failure is detected comprises preserving the first portion of the preserved data at a first data register and preserving the second portion of the preserved data at a second data register.
 19. The flash memory device of claim 18, wherein the second data register is adapted to receive external data during operation.
 20. The flash memory device of claim 18, wherein programming a third location of the memory array with a second portion of the preserved data comprises transferring the second portion of the preserved data from the first data register to the second data register and from the second data register to the third location of the memory array.
 21. A mass storage device comprising: an array of memory cells; a first latch connected to the array of memory cells; a second latch connected to the first latch and to the array of memory cells; and a command control circuit for controlling access to the array of memory cells, wherein the command control circuit is adapted to perform a method, the method comprising: detecting a programming failure at a first location of the array of memory cells; preserving data at the first and second data latches and at the first location of the array of memory cells when the program failure is detected; programming a second location of the array of memory cells with a first portion of the preserved data from the first data latch; programming a third location of the array of memory cells with a second portion of the preserved data from the second data latch; and combining, at the second location of the array of memory cells, the first portion of the preserved data programmed in the second location of the array of memory cells with a third portion of the preserved data from the first location of the array of memory cells.
 22. The mass storage device of claim 21, wherein, in the method, programming a third location of the array of memory cells with a second portion of the preserved data from the second data latch comprises: transferring the second portion of the preserved data from the second data latch to the first data latch; and transferring the second portion of the preserved data from the first data latch to the third location of the array of memory cells. 